<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <script src="/js/vue.js"></script>
</head>
<body>
  <div id="app">
      <p>{{msg}}</p>
  </div>
<br>
  <script>
   let app = new Vue({
   el:"#app",
   data:{ 
     firstName:'Foo',
     lastName:'Bar'
      },
    watch:{
        firstName:function(){
            this.fullName=val+''+this.lastName;
        },
        lastName:function(){
            this.fullName=this.firstName+''+val;
        }
    },
    computed:{
        fullName:{
            get:function(){
                return this.firstName+''+this.lastName;
            }
        },
        set:function(newValue){
            var names=newValue.split('');
            this.firstName=names[0];
            this.lastName=names[names.length-1];
        }
    }
    })
  </script>
</body>
</html>